Chemical milling of magnesium metal and magnesium alloys



United States Patent 3,134,702 CHEMIQAL MILLING (NF MAGNESIUM METAL AND MAGNESIUM ALLOYS Herbert K. De Long and Donald E. Ritzema, both of Midland, Mich, assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Filed Aug. 17, 1960, Ser. No. 50,035

12 Claims. (Cl. 15618) The invention provides a novel aqueous acidic solution and method employing such solution for the controlled removal of metal from a surface of a magnesium metal or magnesium-base alloy article. Both magnesium metal and magnesium-base alloys wherein the magnesium content is at least about 75 percent by weight will hereinafter be referred to as magnesium.

In the production of extruded, rolled, molded, cast, or fabricated magnesium articles, it is often desirable to reduce the dimensions of the magnesium articles and particularly to reduce the cross-sectional area of members comprising the articles.

A method of uniformly reducing the dimensions of magnesium articles and more particularly reducing the cross-sectional area of members thereof by submerging the articles in a bath of specified composition accompanied by movement of either the submerged article or of the bath relative to each other is described in our c0- pending application S.N. 596,390, filed July 9, 1956, now US. Patent No. 3,039,909. Such process is often referred to broadly as chemical milling. In that application, a highly eflicient and economical method of producing uniformly dimensioned magnesium articles without undesirable concomitant formation of pits, ridges, striations, raised areas, rounded corners, and the like, associated to a more-or-less extent with theretofore known dimensioning methods is described. The method therein described consists essentially of submerging the magnesium article in an aqueous acidic bath comprising from about 0.70 to 25.0 percent by weight of nitric acid and from about 3 percent by weight to the saturation point in said bath of a nitrate selected from the nitrates of ammonium, alkali metals, and alkaline earth metals, and the balance water. The method of dimensioning magnesium articles according to S.N. 596,390 was a definite advancement in the art.

However, we have now discovered a novel aqueous composition and method employing such composition, for chemically milling or uniformly dimensioning magnesium articles which removes metal from the surfaces of the magnesium article at an even more uniform rate which is more easily controlled and which results in a smoother finished article than is attainable by the practice of the invention set out in SN. 596,390.

The present invention, accordingly, is an improved method of dimensioning or chemically milling magnesium articles. It employs a novel aqueous composition consisting essentially of phosphoric acid, a nitrate salt selected from the class consisting of nitrates of ammonium, alkali metals, alkaline earth metals such as magnesium, and mixtures thereof, an aliphatic polyhydric alcohol, and the balance substantially water.

The proportional parts of the ingredients of the novel composition of the invention may vary over a rather wide range. They may be employed as follows:

Between and 85 parts by volume of aqueous phosphoric acid, containing at least 85 percent H PO in water, or equivalent amount of such acid and water;

Between 5 and 25 parts by volume of the polyhydric alcohol, but not in excess of a total of 100 parts of the acid and alcohol;

Additional water, if necessary, to make parts of g V the composition by volume; and

Between 1 and 45'percent, by weight of the aqueous composition so made, of the nitrate salt.

The preferred proportional parts of the ingredients of the composition of the bath are as follows:

Between 20 and 30 parts by volume of aqueous phosphoric acid, calculated as 100 percent H PO Between 10 and 17 parts by Volume of the polyhydric alcohol;

Sufficient water to make 100 parts by volume; and

Between 3 and 5 percent, by Weight of the aqueous composition so made, of the nitrate salt.

The preferred temperature range of the treating com position employed in the practice of the invention is between 70" and 90 F. When the ingredients of the bath are employed within the preferred range, as set forth above and within the preferred temperature range, a metal removal rate is obtained of between 0.0005 and 0.002 inch per exposed surface per minute.

The invention is carried out by admixing theingredients of the composition in any order of addition in a suitable vessel or vat and continuing agitation for a time sufiicient to render the aqueous composition substantially uniform and thereafter immersing the magnesium article to be chemically dimensioned or milled therein for a period of time sufficient to remove the thickness of metal desired. It is preferred that the article be moved relative to the bath during at least a part of the period of immersion.

One mode of carrying out the invention is to fill a suitable vat or vessel provided with a stirring means with a measured volume of water, admix therewith the desired volume of the polyhydric alcohol which is com monly either a glycol containing from 2 to 5 carbon atoms or glycerine, then admix phosphoric acid, usually having a concentration of at least 85 percent by weight, to provide the desired concentration of the acid in the aqueous composition, and then admix sufiicient soluble nitrate salt to provide the desired concentration of the N0 radical.

The magnesium article to be degreased, if necessary, as by immersing it in an aqueous alkaline solution, e.g. one containing about 3 weight ounces of sodium carbonate and about 2 ounces of sodium hydroxide per gallon of water, at a temperature of between 190 and 212 F. for between 3 and 10 minutes, removing it therefrom and rinsing it with water. The thus cleaned magnesium article is then immersed in the aqueous treating composition of the invention for suflicient time to remove the desired amount of metal therefrom at a temperature of between 60 and 180 F. It is then removed therefrom and rinsed with water. Either the article being treated or the treating composition should be maintained in a state of motion for at least a part of the period of immersion in the composition, relative to the other for satisfactory results.

When it is desired that certain portions of the metal article retain their original dimensions, such portions may be masked off or stopped off by a suitable masking material. Such masking material may be any material which is substantially impervious to the chemical milling solution, e.g. it may be any one of a number of acidresistant resin, such as polyvinylchloride as suggested in column 2, line 68, of US. Patent 3,039,909, or fibrous materials. Any of such materials prevents access of the aqueous acid solution to the thus masked portions which thereby retain their original sizewhile the unmasked portions are appropriately reduced in size by the action of the treating composition.

A series of test runs was made, some for the purpose of illustrating the invention and others for the purpose milled is usually first of comparison. The test runs made for comparison were in accordance with existing practices considered to be among those producing best results by methods known prior to the present invention. Those runs which are illustrative of the invention are designated examples and those not illustrative of the invention and for comparison are designated comparative runs.

In all test runs, the same general procedure was followed. The order of addition of each of the compoknown practices Were in some instances rough and lackluster (satiny).

An examination of Examples 1, 2, 3 shows more specifically the greater uniformity of metal removed in accordance with the practice of the invention than in accordance with known practice. In each of the examples of the invention the average difierence between the maximum and minimum thicknesses of metal removed (calculated by adding the maximum and minimum values nents, when employed, was as follows: The water was together and dividing the sums obtained by 2) was placed in a suitable container. The acid was first added 0.0005". In contrast thereto, the average difference befollowed by the polyhydroxy alcohol, to make 100 parts tween the maximum and minimum thicknesses of metal by volume. The nitrate salt, calculated in percent by removed in experimental runs A-F, which are illustraweight of the aqueous 801101011, Was th adml d th tive of prevailing conventional practice (similarly cal- Wlthculated as in the examples of the invention), were 0.003,

a p p thegrnagngswrnnalloy set out 111 the table, 0.003, 0.0035, 0.003, 0.003 and 0.004 inch, respectively. having isrze of 1 x 3 x 1 W61: pr g il f -2 3 The average of these values is 0.00325. To show the i an dalme 9 x 3 W an 3 Immerse extent to which uniformity is attained by the practice of m q mice 16 mven Ion In o aqueous the invention in contrast to that attained by the runs illussolutions set out in the table below, while continuing to 20 rating prior practice, 0.00325/0.0005=6.5, 1.e. the uniagltate the Sohmcn mlldly by means of a blade-type formit ttained b the ractice of the invention ma be agitator or air injection, at the temperature and for the 6 5 y t) g u ad b u 1 f time shown in the table. The rate of metal removal, the Sm lmes a a y 16 emp oymen O uniformity of the metal layer removed, and the appearthe est w i ance of the thus chemically milled surfaces are also set The Practlca mvfinflon offefs a number R 'f out in the table. In the table, the percent of all ingrevantages chenllcany 1111111118, etchlng, 0f dlmenslonmg dients in the treating composition is by volume except of mag artlcles- Among such advantages are! the acid salt (when employed) and the nitrate salt which are in percent by weight of the aqueous composition. getter g g i f The maximum and minimum thicknesses of metal re- Smoo mg g Sur g d d moved per minute in the test runs as Set out in the table, Sharper relief, 1.e. substant a a sence o roun e corners,

from each sample panel was ascertained. The average Greater umfonmty thlckness of metal rem?ved; value f h maximum d i i hi k was Insignificant undercutting unless an unusually thick layer then calculated by dividing the sum of the maximum and f metal is sought to be removed, andthen such n minirnum thicknesses by 2. cutting as occurs is substantially uniform.

Rate of Metal Removed Per Uniiorm- Test Time Temp., Metal Surface, in. ity of Appearance of Dried No. Alloy Chemical Milling Solution 1 (Min) F. Removal, Chemical Treated Surface ir1./min. Milling l\Iaximu m Minimum in.

M312 nitric acid gth); 88% 20 81-89 0.001 0. 0235 0. 0175 i0.003 Bright and smooth.

water, by volume 45% Mg(NO3)z. 6H20, by weight.

13 AZ31 14% nitric acid (70% strength); 80% 20 32-02 0. 001 0. 023 0.017 $0.003 Do.

watelr,t by volume NaNOa, by weigi O AZ31 18% NaHSO4; 5% citric acid; 3% 10 85-90 0.003 0.032 0.025 i0. 0035 Bright but hada monochloroacetic acid; 74% water, satin appearance. by volume; 10% NaNOg, by Weight.

1 AZ31 30% phosphoric acid (100% strength); 10 80-82 0. 00005 0.011 0.010 $0. 0005 Bright and smooth.

10% ethylene glycol; 54% water, by volume; 5% NaNOg, by weight.

2 AZSl 30% phosphoric acid (100% strength); 24 80-80 0.0008 0.0105 0.0185 310.0005 Do.

17% glyccrine; 53% water, by volume; 5% NaNOg, by weight.

D 111531 3 12% nitric acid; (70% strength); 88% 20 81-95 0.001 0.0235 0.0175 i0. 003 Bright but core area water, by volume; Mg(NO3)g. rough. GHzO, by Weight.

HK31 14% nitric acid strength); 20 82-02 0.001 0.023 0.017 i0. 003 Do.

water, by volume; 40% NaNOa, by weight.

F HK31 5% citric acid, 3% monochloroacetic 8 80-85 0.003 0.028 0.020 $0.004 Satin bright.

acid; 92% water, by volume; 18%

NaHSO4, by weight, 10% NaNO3, by weight.

3 1IK31 30% phosphoric acid (100% strength); 40 80-82 0.0005 0.0205 0. 0195 :110. 0005 Bright and smooth.

17% glycerine; 53% water, by volume; 5% NaNOa, by weight.

l The liquid ingredients are on a 100 percent by volume basis; the nitrate salt is in percent by weight of the liquid ingredients. 1 AZ31, composition in percent by weightzabout 3 Al, 1 Zn, 0.4 Mn, and balance Mg.

3 HK31, composition in percent by weight: about 3 Th, 07 Zr, and balance Mg.

4 Obtained by dividing the sum of the maximum and minimum thickness of metal removed, in inches, by 2.

Reference to the table shows that superior results were obtained in both better control of the rate of removal and in uniformity of the thickness of the metal removed when the magnesium test panels were chemically milled in the composition of the invention, in accordance with the improved method employing such composition. The table further shows that when the composition of the invention was employed, the appearance of the finished surface was always bright and smooth whereas panels treated with other compositions selected from the Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. An aqueous acidic treating bath for chemically dimensioning magnesium articles comprising by volume between 5 and parts of phosphoric acid, having a concentration of at least 85 percent H PO between 5 and 25 parts of an aliphatic polyhydric alcohol (the total of the acid and alcohol not exceeding parts); balance water, as needed, to make 100 parts by volume;

best 75 and between 1 and 45 percent by weight of the composition so made of a nitrate salt selected from the class consisting of the nitrates of ammonium and alkali and alkaline earth metals.

2. An aqueous acidic treating bath for chemically dimensioning magnesium articles comprising by volume phosphoric acid in amount equivalent to between 20 and 30 parts of 100 percent acid; between and 17 parts of an aliphatic polyhydric alcohol; suificient water to make 100 parts by volume; and between 3 and 5 percent by weight of the composition so made of a nitrate salt selected from the class consisting of the nitrates of ammonium and alkali and alkaline earth metals.

3. The bath of claim 2 wherein the aliphatic polyhydric alcohol is selected from the class consisting of aliphatic glycols having between 2 and 5 carbon atoms, glycerine, and mixtures thereof.

4. The bath of claim 3 wherein the aliphatic polyhydric alcohol is ethylene glycol.

5. The bath of claim 3 wherein the aliphatic alcohol is glycerine.

6. The bath of claim 2 wherein the nitrate salt is magnesium nitrate.

7. The bath of claim 2 wherein the nitrate salt is sodium nitrate.

8. The method of chemically dimensioning magnesium article consisting of immersing a substantially grease-free magnesium article to be sized in the bath of claim 1 at a temperature of between 60 and 180 F. while maintaining a relative motion between said bath and said article for at least a portion of the time of immersion, allowing the article to remain thus immersed until the dimensions of said article are reduced to the size desired, removing the article from the bath, and rinsing it with water.

9. The method according to claim 8 wherein said bath is maintained at a temperature of between 70 and 90 F.

10. The method of chemically dimensioning a magnesium article consisting of immersing a substantially grease-free magnesium article to be sized in the bath of claim 2 at a temperature of between and 180 F. while maintaining a relative motion between said bath and said article for at least a portion of the time of immersion, maintaining the article immersed for a time sufiicient to reduce the dimensions of the members of said article to the size desired, removing the so treated article from the bath, and rinsing it with water.

11. The method of selectively chemically dimensioning a magnesium article consisting of masking off the portions of said article which are to remain unreduced in size with a coating material which is impervious to aqueous acidic solutions and immersing the thus partially coated article in the bath of claim 1 at between 60 and 180 F. while maintaining a relative motion between said bath and article for at least a portion of the time of immersion until the unmasked portions of said article are reduced to the desired dimensions, removing the so treated article, and rinsing the article with water.

12. The method according to claim 11 wherein the bath is maintained at a temperature of between and F.

References Cited in the file of this patent UNITED STATES PATENTS 1,977,646 Rowell Oct. 23, 1934 2,393,875 Van Dusen Jan. 29, 1946 2,650,157 Cochran Aug. 25, 1953 2,687,345 Murray Aug. 24, 1954 2,849,297 Wisken Aug. 26, 1958 2,925,332 Standley Feb. 16, 1960 2,980,559 Whitby Apr. 18, 1961 3,008,409 Wentworth et a] Nov. 14, 1961 3,023,138 Easley et a1 Feb. 27, 1962 OTHER REFERENCES Hackhs Chemical Dictionary, Grant, 3rd edition, copyright 1944, page 776. 

1. AN AQUEOUS ACIDIC TREATING BATH FOR CHEMICALLY DIMENSIONING MAGNESIUM ARTICLES COMPRISING BY VOLUME BETWEEN 5 AND 85 PARTS OF PHOSPHORIC ACID, HAVING A CONCENTRATION OF AT LEAST 85 PERCENT H3PO4; BETWEEN 5 AND 25 PARTS OF AN ALIPHATIC POLYHYDRIC ALCOHOL (THE TOTAL OF THE ACID AND ALCOHOL NOT EXCEEDING 100 PARTS); BALANCE WATER, AS NEEDED, TO MAKE 100 PARTS BY VOLUME; AND BETWEEN 1 AND 45 PERCENT BY WEIGHT OF THE COMPOSITION SO MADE OF A NITRATE SALT SELECTED FROM THE CLASS CONSISTING OF THE NITRATES OF AMMONIUM AND ALKALI AND ALKALINE EARTH METALS.
 11. THE METHOD OF SELECTIVELY CHEMICALLY DIMENSIONING A MAGNESIUM ARTICLE CONSISTING OF MASKING OFF THE PORTIONS OF SAID ARTICLE WHICH ARE TO REMAIN UNREDUCED IN SIZE WITH A COATING MATERIAL WHICH IS IMPREVIOUS TO AQUEOUS ACIDIC SOLUTIONS AND IMMERSING THE THUS PARTIALLY COATED ARTICLE IN THE BATH OF CLAIM 1 AT BETWEEN 60* AND 180*F. WHILE MAINTAINING A RELATIVE MOTION BETWEEN SAID BATH AND ARTICLE FOR AT LEAST A PORTION OF THE TIME OF IMMERSION UNTIL THE UNMASKED PORTIONS OF SAID ARTICLE ARE REDUCED TO THE DESIRED DIMENSIONS, REMOVING THE SO TREATED ARTICLE, AND RINSING THE ARTICLE WITH WATER. 